2024: Add reserved syntax

Author: ehuss

src/SUMMARY.md

@@ -55,3 +55,4 @@
   - [Unsafe `extern` blocks](rust-2024/unsafe-extern.md)
   - [Unsafe attributes](rust-2024/unsafe-attributes.md)
   - [Rustdoc combined tests](rust-2024/rustdoc-doctests.md)
+  - [Reserving syntax](rust-2024/reserving-syntax.md)

src/rust-2024/reserving-syntax.md

@@ -0,0 +1,63 @@
+# Reserved unprefixed guarded strings
+
+🚧 The 2024 Edition has not yet been released and hence this section is still "under construction".
+More information may be found in the tracking issue at <https://github.com/rust-lang/rust/issues/123735>.
+
+## Summary
+
+- Unprefixed guarded strings of the form `#"foo"#` are reserved for future use.
+- Two or more `#` characters are reserved for future use.
+
+## Details
+
+[RFC 3593] reserved syntax in the 2024 Edition for guarded string literals that do not have a prefix to future-proof against future language changes. The 2021 Edition [reserved syntax][2021] for guarded strings with a prefix, such as `ident##"foo"##`. The 2024 Edition extends that to also reserve strings without the `ident` prefix.
+
+There are two reserved syntaxes: One or more `#` immediately followed by a [string literal], and two or more `#` characters in a row.
+
+This reservation is done across an edition boundary because of interactions with tokenization and macros. For example, with the following macro:
+
+```rust
+macro_rules! demo {
+    ( $a:tt ) => { println!("one token") };
+    ( $a:tt $b:tt $c:tt ) => { println!("three tokens") };
+}
+
+demo!("foo");
+demo!(r#"foo"#);
+demo!(#"foo"#);
+demo!(###)
+```
+
+Prior to the 2024 Edition, this produces:
+
+```
+one token
+one token
+three tokens
+three tokens
+```
+
+Starting in the 2024 Edition, the `#"foo"#` line and the `###` line now generates a compile error because those forms are now reserved.
+
+[2021]: ../rust-2021/reserving-syntax.md
+[string literal]: ../../reference/tokens.html#string-literals
+[RFC 3593]: https://rust-lang.github.io/rfcs/3593-unprefixed-guarded-strings.html
+
+## Migration
+
+The [`rust_2024_guarded_string_incompatible_syntax`] lint will identify any tokens that match the reserved syntax, and will suggest a modification to insert spaces where necessary to ensure it continues to be parsed as separate tokens.
+
+The lint is part of the `rust-2024-compatibility` lint group which is included in the automatic edition migration. In order to migrate your code to be Rust 2024 Edition compatible, run:
+
+```sh
+cargo fix --edition
+```
+
+Alternatively, you can manually enable the lint to find macro calls where you may need to update the tokens:
+
+```rust
+// Add this to the root of your crate to do a manual migration.
+#![warn(rust_2024_guarded_string_incompatible_syntax)]
+```
+
+[`rust_2024_guarded_string_incompatible_syntax`]: ../../rustc/lints/listing/allowed-by-default.html#rust-2024-guarded-string-incompatible-syntax